The invention relates to a system for controlling the fluid level in a vessel. Among numerous applications for such a system is the maintenance of the water level in a nuclear reactor.
In well-known commercial boiling water nuclear power reactors, for example as used in the Dresden Nuclear Power Station near Chicago, Ill., a core of fuel material contained in a pressure vessel is submerged in a fluid, such as light water, which serves both as a working fluid and a neutron moderator.
The water is circulated through the core whereby a portion thereof is converted to steam. The steam is taken from the pressure vessel and applied to a prime mover such as a turbine. The turbine exhaust steam is condensed and, along with any necessary make-up water, returned to the pressure vessel as feedwater. Reactor power level is controlled by a system of control rods, containing neutron absorber material, which are selectively insertable into the core. Further information on nuclear reactors may be found, for example, in "Nuclear Power Engineering", N. M. El-Wakil, McGraw-Hill Book Company, Inc., 1962.
Nuclear reactors are provided with a protection system which monitors various aspects of reactor operation including water level. If a fault develops in the water level control system and it fails to maintain a predetermined water level in the vessel, the water level becomes "out of limits", either too high or too low, and the protection system "scrams" the reactor that is, it causes rapid insertion of the control rods whereby the reactor is shut down automatically. Such reactor shutdowns are undesirable for a variety of reasons. Even if the fault is corrected readily, restart of the reactor is a relatively lengthy process. Meanwhile, customers may suffer a loss of power or the power must be supplied from other, usually more costly, sources.
Thus an object of the invention is an improved liquid level control system which is tolerant of a failure of components therein. Another object is a level control system having redundant control channels with means for switching control automatically to another channel upon an excursion of the liquid level beyond prescribed limits.